1. Field of the Invention
The present invention relates to a method and system for treating wastewater containing organic compounds. The present invention combines the advantages of anaerobic and aerobic systems for disposing organic pollutants in the wastewater, and integrates both systems with the membrane separation system to increase system efficiency and lower the costs of system installation.
2. Description of Related Art
Membrane biological treatment system is one the most popular technologies used in wastewater treatment in recent years. Such system employs membrane as separation medium, in which wastewater is first decomposed by microbes and then passed through a membrane by of the driving force of vacuum pressure difference to separate solids from liquids. The resulting permeate that passes through the membrane is the treated water, while sludge and other solid matters are entrapped in the membrane separation reactor and discharged. Reverse washing or chemical cleaning is employed to remove pollutants deposited on the surface of the membrane to prolong the membrane working life.
Membrane filtration is a key element of the membrane biological treatment system. Microfiltration may be generally classified into dead-end filtration and cross-flow filtration. Dead-end filtration is similar to the conventional cake filtration where the flow of the suspension is perpendicular to the membrane surface, while solid substances are intercepted on the membrane surface and form filter cake. The thickness of filter cake is increased with the filtration volume of suspension. The flow of permeates through the membrane decreases as the filter cake thickens, which relies on the working of reverse washing cycle for cake removal from the system. In the case of cross-flow filtration, the suspension flow is parallel to the membrane surface where the cross-flow rate of suspension produces shear stress to eliminate the formation of filter cake. When the cumulative effect of the filter cake reaches equilibrium with the elimination effect, the filter cake would have constant thickness. In cross-flow filtration, there must be a rate component parallel to the membrane surface. The movement of the membrane itself or the disturbance of air bubbles can obtain such effect. For the purpose of preventing fouling so as to generate maximum permeation throughput, membrane biological treatment system typically adopts cross-flow filtration for solid-liquid separation.
In considering whether the membrane biological treatment system is economically feasible, the removal of fouling or scaling on membrane surface to prolong membrane life and lower energy loss is a key factor. Fouling refers to solid matters such as sludge, ultra fine colloid particles, and organic matters adsorbed to or settled on membrane that adds to the permeation resistance. The causes of fouling involve physical and chemical mechanisms as well as the phenomenon of concentration polarization. Scaling refers to the formation of metal crystals such as oxides, carbonates and phosphates from metals or heavy metals. For example, in the process of anaerobic reaction, the concentration of carbonate in water increases, and if Ca, Mg, Fe or other heavy metals are present in the water, the scaling of metal carbonate on membrane surface would obstruct the membrane. Serious fouling or scaling on membrane would greatly reduce the treatment throughput. Thus it is necessary to remove scaling periodically in the process of the treatment system.
In comparison with conventional activated sludge process, membrane biological treatment system requires relatively less space, and is easy to operate and maintain. The sludge entrapped in the system may be held for a long time to facilitate the removal of special or not readily biodegradable pollutants and the biological sludge can be intercepted completely. Moreover, the system does not require a settler unit, thereby saving space and increasing treatment efficiency. It is also capable of entrapping polymers that are difficult to break down. Therefore, the membrane biological treatment system is particularly suitable for small and medium scale treatment of special pollutants.
Due to the advances in membrane production technology in recent years, the membrane usage life is extended, and the loading level of membrane treatment system has increased from laboratory application level to plant applications level with capacity of 10,000 m3/d. The main membrane biological treatment system used Currently is aerobic membrane biological treatment system. But the installation cost of aerobic membrane treatment system has been persistently high due to the high cost of membrane. This invention combines both aerobes and anaerobes in the membrane biological treatment system to address the problems aforesaid.